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Enhanced photoelectrochemical water splitting on Pt-loaded TiO2 nanorods array

thin film

Fangfang Wang a, Zhi Zheng b, Falong Jia a,a College of Chemistry, Central China Normal University, Wuhan 430079, PR Chinab Institute of Surface Micro and Nano Materials, Xuchang University, Xuchang 461000, PR China

a b s t r a c ta r t i c l e i n f o

Article history:

Received 9 October 2011Accepted 6 December 2011

Available online 21 December 2011

Keywords:

TiO2Photoelectrochemical water splitting

Nanocomposites

Thin films

We demonstrate that appropriate Pt loading could significantly enhance the ability of TiO2 nanorods array

thin film (TNTF) electrode to photoelectrochemically split water under solar light. The TiO2 nanorods array

thin film was directly grown on fluorine-doped tin oxide glass through hydrothermal reaction. And platinum

(Pt) nanoparticles were deposited uniformly on the surface of TiO2 nanorods by a convenient sputtering

method. The Pt-loaded TNTF sample was highly stable during the photoelectrochemical water splitting process.

Itsactivity didnot decreaseafter 50continuous potential scans.Thisstudyrevealsthat thePt loadedTiO2nanorods

array thin film electrode is promising for the photoelectrochemical water splitting to generate hydrogen.

2011 Elsevier B.V. All rights reserved.

1. Introduction

As one of the sustainable sources of energy, hydrogen fuel has been

studied for many years for its unique advantages such as clean exhaustandlightweight [1]. Electrolysisof water provides a clean andsimplified

way to get highly pure hydrogen. The photocatalytic production of H2via water splittingover semiconductor photocatalyst attracted consider-

able attention and many semiconductor catalysts have been widely

studied [2]. Among the exploited catalysts, titanium dioxide (TiO2) is

one of the most popular catalysts for water splitting because of its high

chemical stability, non-toxicity, as well as low cost [3]. One dimensional

TiO2 was found to exhibit superior photoelectrochemical performances

than TiO2 nanoparticles [4]. The reason is that one dimensional nano-

structure could promote the electron transport rate, and also accelerate

the ion diffusion at the semiconductorelectrolyte interface [5].

Pure TiO2 has very low photoresponse due to its wide band gap

(~3.2 eV). Therefore, extensive efforts have been made to enhance

its photoactivity through modifying the band structures by dopant

materials [6] and sensitizing with semiconductor quantum dots [7].

It is reported that Pt modification could improve the photoactivity

of TiO2 [8], because the deposition of Pt on TiO2 not only inhibits

electronhole pair recombination, but also widens the spectral re-

sponse to longer wavelengths [9]. By now, various TiO2 photocata-

lysts loaded with Pt have been prepared for the degradation of

organic pollutants [10]. Although Pt-loaded TiO2 nanoparticles and

nanotubes were used for photoelectrochemical water splitting [11],

Pt-loaded TiO2 nanorods were seldom studied.

In this communication, we report that the Pt-loading could signif-

icantly enhance the ability of TiO2 nanorods array thin film (TNTF) tophotoelectrochemically split water at light and discuss the reason for

this enhancement.

2. Experimental

2.1. Preparation of Pt-loaded TiO2 nanorods thin film

TheTiO2 nanorod arrays were grown through hydrothermal reaction

according to Liu's method [12]. Typically, 0.11 mL of titanium butoxide

(97%, Aldrich) was dropped into the solution with 4.7 mL of deionized

water and 4.7 mL of concentrated hydrochloric acid (AR, 37wt.%)

under vigorous stirring. And the solution was transferred into a 23 mL

Teflon-lined stainless steel autoclave. The well-cleaned fluorine doped

tin oxide (FTO glass, NSG, Japan) coated glass slides were put into the

Teflon reactor and the autoclave was kept at 150 C in an oven for

20 h. After cooling down to room temperature naturally, the FTO glass

was taken out, washed with deionized water and dried at 50 C. Pt

wassputtered onto TiO2 nanorods thinfilms byusing anautofine coater

(JFC-1600, Japan)with current of 20 mA andchamber pressureless than

105 Pa. The as-prepared sample was directly annealed in a furnace at

450 C for 2 h. The samples were characterized by field emission scan-

ning electron microscopy (FESEM, JEOL-6700F), transmission electron

microscopy (TEM/HRTEM, JEOL JSM-2010), X-ray diffraction (XRD,

MPD 18801, Cu K) and UVvisible absorption spectroscopy (S-3100,

SCINCO).

Materials Letters 71 (2012) 141144

Corresponding author. Tel./fax: +86 27 6786 7953.

E-mail address: fljia@mail.ccnu.edu.cn (F. Jia).

0167-577X/$ see front matter 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.matlet.2011.12.063

Contents lists available at SciVerse ScienceDirect

Materials Letters

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t l e t

http://dx.doi.org/10.1016/j.matlet.2011.12.063http://dx.doi.org/10.1016/j.matlet.2011.12.063http://dx.doi.org/10.1016/j.matlet.2011.12.063mailto:fljia@mail.ccnu.edu.cnmailto:fljia@mail.ccnu.edu.cnhttp://dx.doi.org/10.1016/j.matlet.2011.12.063http://www.sciencedirect.com/science/journal/0167577Xhttp://www.sciencedirect.com/science/journal/0167577Xhttp://dx.doi.org/10.1016/j.matlet.2011.12.063mailto:fljia@mail.ccnu.edu.cnhttp://dx.doi.org/10.1016/j.matlet.2011.12.063

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2.2. Photoelectrochemical measurements

All electrochemical experiments were performed on an electrochem-ical workstation (CHI660B, CHI Instruments, Shanghai). A standard

three-electrode configuration was set up with TNTF/FTO glass as the

working electrode (0.5 cm2 area), a platinum foil as counter electrode

and a saturated Ag/AgCl reference electrode. The photoelectrochemical

measurements were conducted in 0.05 mol/L KOH aqueous solution illu-

minated under 1.5 AM solar simulator (Shanghai lan sheng Co., 500 W

xenon lamp). The intensity of light on the sample was 200 mW/cm2.

3. Results and discussion

Fig. 1 shows the XRD patterns of the resulting TNTF samples withand without annealing treatment. The peaks located at 36.0 and 62.6

corresponded to the (101) and (002) crystal planes of rutile titania

(JCPDS file No. 76-1939), respectively. The peak strength of the sample

was enhanced greatly after annealing treatment at 450 C for 2 h, sug-

gesting that the annealing treatment resulted in better crystallization.

Fig. 2 shows the surface morphologies of TNTF sample with 20 s Pt

sputtering. Uniform nanorods were found to grow on the FTO

Fig. 1. XRD patterns of the as-synthesized samples (a) and after annealing treatment at 450 C for 2 h (b). The peaks labeled with mark o resulted from FTO substrate.

Fig. 2. SEM (a, b), TEM (c), and HRTEM (d) images of TNTF samples with Pt sputtering for 20 s.

142 F. Wang et al. / Materials Letters 71 (2012) 141144

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substrate with ~100 nm in width and each nanorod consisted of bun-

dles of smaller nanorods (Fig. 2a and b). The TEM image (Fig. 2c) re-

veals that the nanorod bundles consist of radially aligned nanorods of

~20 nm in diameter with narrow gaps among the nanorods. It can be

seen that Pt nanoparticles with size of ~4 nm were deposited uniformly

onthe surface ofTiO2 nanorods.The HRTEM resultgives a better view of

the sputtered Pt nanoparticles (see the circled area) with lattice fringe

of 0.22 nm, which matches with the (111) plane of Pt. And the lattice

fringes of TiO2 nanorods could also be observed and the value of0.29 nm can be assigned to the (001) planes of rutile TiO2. EDS analysis

showed that the compositions of Pt in the samples were 0.12, 0.18 and

0.25 at.% with Pt sputtering time of 10 s, 20 s and 30 s, respectively.

Fig. 3 shows the linear sweep voltammograms recorded with differ-

ent TNTF electrodes. All these electrodes exhibited very low current

densities in the scale of A/cm2 in dark (Fig. 3A). The anodic currents

dramatically increased to the range of mA/cm2 after shining simulated

solar light (Fig. 3B). The enhancement of current density is the lowest

for the unannealed TNTF electrode among all the samples. The current

density of annealed TNTF electrode at 1.0 V was about five times that

of the unannealed one and could be further enhanced by Pt loading.

10 second Pt sputtering only slightly improved the responsive current

density. With the sputtering time prolonging to 20 s, thecurrent densi-

ty at 1.0 V (3.8 mA/cm2) increased nearly four times that (0.96 mA/

cm2) of bare annealed TNTF electrode. However, the current density

tended to decrease with further prolonging the sputtering time.

For TiO2-based photoelectrochemical system, quick recombination

between photogenerated electrons and holes is the major factor to de-

crease the photoactivity. When the surface of TiO2 nanorods was loaded

with platinum, the highly dispersed Pt nanoparticles not only facilitate

the exciton separation, but also enhance photogenerated electrons

transport. As shown in Fig. S1,the loading of Pt could increase theoptical

absorption in the visible region effectively. No saturation of current at

more positive potentialfor allPt-loaded TNTF samples indicatesefficient

charge separation in nanorods upon illumination. Therefore, Pt-loading

is effective for the improvement of the photoelectrochemical perfor-

mance of theTNTF electrode. Thereason forthe decreaseof current den-

sity with 30 second Pt loading may be attributed to the fact that excess

Pt coating hindered the exposure of TiO2 to the electrolyte.

To test the stability of Pt-TNTF electrode during the photoelectro-chemical process, constant potential of 0.5 V was applied to the sample

with the light on and off repeatedly. The It curve in Fig. 3C displays a

very low dark current and a steady photocurrent quickly upon illumina-

tion of light. Even after a long time (e.g. 1 h) of onoff light cycle, the

photoresponsive property of Pt-TNTF electrode almost remained the

same. In Fig. 3D, the IV curve after 50 times continuous potential scan

under light almost overlapped with that of the first one, indicating that

the Pt-loaded TNTF electrode is highly stable during the photoelectro-

chemical water splitting process.

4. Conclusions

In summary, we demonstrate that an appropriate Pt loading could

significantly enhance the ability of TiO2 nanorods array thin film elec-

trode to photoelectrochemically split water under solar light. The

TiO2 nanorods array thin film was directly grown on fluorine-doped

tin oxide glass. The Pt-loaded TNTF/FTO sample was highly stable in

the photoelectrochemical water splitting process. This study suggests

that Pt loaded TNTF/FTO electrode is promising for the photoelectro-

chemical water splitting to generate hydrogen.

Fig. 3. Liner sweep voltammograms in dark (A) and light (B) with different samples. (a) and (b) are TNTF samples before and after annealing. (c), (d) and (e) are the annealed TNTF

samples with Pt sputtering for 10, 20 and 30 s. (C) and (D) are the I

t curve at 0.5 V, and 1st and 50th liner sweep voltammograms in light with sample (d) as electrode.

143F. Wang et al. / Materials Letters 71 (2012) 141144

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Supplementary materials related to this article can be found online

at doi:10.1016/j.matlet.2011.12.063.

Acknowledgments

This work was supported by the National Science Foundation of

China (Grant 21073070), the self-determined research funds of

CCNU from the colleges' basic research and operation of MOE

(CCNU11A02006), and the National Science Foundation of HubeiProvince (Grant 2010CDB04004).

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